Local chromatin architecture is generally recognized as an important factor in the regulation of gene expression. The architecture of chromatin, a protein-DNA complex, is strongly influenced by post-translational modifications of the histones which are the protein components. Reversible acetylation of histones is a key component in the regulation of gene expression by altering the accessibility of transcription factors to DNA. In general, increased levels of histone acetylation are associated with increased transcriptional activity, whereas decreased levels of acetylation are associated with repression of gene expression [Wadem P. A. Hum. Mol. Genet. 10, 693-698 (2001), De Ruijter A. J. M. et al, Biochem. J., 370, 737-749 (2003)]. In normal cells, histone deacetylases (HDACs) and histone acetyltransferase together control the level of acetylation of histones to maintain a balance. Inhibition of HDACs results in the accumulation of acetylated histones, which results in a variety of cell type dependent cellular responses, such as apoptosis, necrosis, differentiation, cell survival, inhibition of proliferation and cytostasis.
Inhibitors of HDAC have been studied for their therapeutic effects on cancer cells. For example, suberoylanilide hydroxamic acid (SAHA) is a potent inducer of differentiation and/or apoptosis in murine erythroleukemia, bladder, and myeloma cell lines [Richon V. M. et al, Proc. Natl. Acad. Sci. USA, 93: 5705-5708 (1996), Richon V. M. et al, Proc. Natl. Acad. Sci. USA, 95: 3003-3007 (1998)]. SAHA has been shown to suppress the growth of prostate cancer cells in vitro and in vivo [Butler L. M. et al, Cancer Res. 60, 5165-5170 (2000)]. Other inhibitors of HDAC that have been widely studied for their anti-cancer activities are trichostatin A (TSA) and trapoxin B [Yoshida M. et al, J. Biol. Chem., 265, 17174 (1990), Kijima M. et al, J. Biol. Chem., 268, 22429 (1993)]. Trichostatin A is a reversible inhibitor of mammalian HDAC. Trapoxin B is a cyclic tetrapeptide, which is an irreversible inhibitor of mammalian HDAC. However, due to the in vivo instability of these compounds they are less desirable as anti-cancer drugs. Recently, other small molecule HDAC inhibitors have become available for clinical evaluation [U.S. Pat. No. 6,552,065]. Additional HDAC inhibiting compounds have been reported in the literature [Bouchain G. et al, J. Med. Chem., 46, 820-830 (2003)] and patents [WO 03/066579A2, WO 01/38322 A1]. The in vivo activity of such inhibitors can be directly monitored by their ability to increase the amount of acetylated histones in the biological sample. HDAC inhibitors have been reported to interfere with neurodegenerative processes, for instance, HDAC inhibitors arrest polyglutamine-dependent neurodegeneration [Nature, 413(6857): 739-43, 18 Oct. 2001]. In addition, HDAC inhibitors have also been known to inhibit production of cytokines such as TNF, IFN, IL-1 which are known to be implicated in inflammatory diseases and/or immune system disorders. [J. Biol. Chem. 1990; 265(18): 10230-10237; Science, 1998; 281: 1001-1005; Dinarello C. A. and Moldawer L. L. Proinflammatory and anti-inflammatory cytokines in rheumatoid arthritis. A primer for clinicians. 2nd Edition, Amergen Inc., 2000].
Nevertheless, there is still a need to provide further HDAC inhibitors that would be expected to have useful, improved pharmaceutical properties such as anti-cancer agents.